Method and device for coating a surface with a plastic film

ABSTRACT

Method and apparatus for coating the surface of an object, especially the surface of a car bumper ( 1 ) with a plastic film ( 2 ). During and/or after application of the plastic film to the surface, the coating is irradiated with infra-red radiation in order to heat at least parts of the coating and/or optionally residual moisture between the surface and the plastic film ( 2 ). This enables slight inequalities of the plastic film to be compensated, the adhesive coating is distributed in a homogeneous manner and the optionally residual moisture is collected in the form of bubbles following infrared radiation. Defects or irregularities in the coating can thus be identified during or shortly afterwards.

BACKGROUND OF THE INVENTION

The invention relates to the process of coating a surface with a thinsheet of plastic, in particular the surface of an automobile bumper. Inparticular, during such a surface coating process, the surface and/orthe plastic sheet is moistened so that after the plastic sheet has beenapplied, the surface in at least some places is separated from theplastic sheet by a film of moisture, which is subsequently at leastpartially removed by a squeegee. The invention further relates toquality assurance during the implementation of a method for coating asurface with a thin plastic sheet, in particular the surface of anautomobile bumper.

For various reasons, it is a common current practice to coat thesurfaces of objects with thin sheets of plastic. For example, suchplastic sheets protect vulnerable surfaces from becoming scratched andfrom the action of chemicals, and they can also create colourimpressions by interference effects or can make the surface more shiny.In the automobile industry, for instance, it is customary to providecars with bumpers, the colour of which is coordinated with the colour ofthe body paint. The bumpers are made of a plastic material of thedesired colour which, however, often is relatively soft and flexible, sothat the surface readily becomes scratched as a result of everyday wearand tear. Furthermore, the scratches on a smooth, preferably shiny,lacquer-like plastic surface are considerably more clearly visible thanthey would be on matte, roughly textured surfaces.

A known means of coating a surface with a thin sheet of plastic, inparticular the surface of a car bumper, is to moisten the surface and/orthe plastic sheet so that after the sheet has been applied, the surfaceis separated from the sheet, at least in places, by a film of moisture.The liquid that forms the film of moisture can, in particular, be arinsing solution. The moisture film permits the plastic sheet to bepositioned or repositioned on the surface in exactly the desired manner.After the plastic sheet has been positioned at the correct place or inthe correct region on the surface, the moisture film is at leastpartially, preferably almost completely, removed by means of a squeegee.

Some of the moisture is usually bound to the surface and/or to theplastic sheet. Another portion can enter into combination with anadhesive that has likewise been disposed between the plastic sheet andthe surface. In many cases the moisture actually serves to activate theadhesive, so that an effective and permanent adhesion is achieved.

However, if most of the superfluous moisture is not squeezed out, theresult can be distinctly visible irregularities in the coating. Inparticular, over a period of several hours moisture contents that hadbeen bound in one of the ways described above, or had been relativelyuniformly distributed over the surface, can collect and formmoisture-filled blisters. This presents a major problem for automobilemanufacture, particularly in the case of the currently customaryjust-in-time delivery of fittings. In some circumstances large numbersof, for example, a car bumper must be produced and delivered in arelatively short time, in which case any defects in the coating do notbecome apparent until after the bumpers have been delivered or evenmounted on the cars.

Another problem is presented by irregularities in the plastic sheetand/or in the layer of adhesive that has been applied to the sheet or tothe surface before the coating process is begun. In particular, thethickness of the plastic sheet or the consistency of the plasticmaterial may be nonuniform. Such irregularities in the plastic sheetand/or an adhesive layer often do not become visible until the plasticsheet has been applied to the surface of the object to be coated. Thereason is, in particular, that the plastic sheet in many cases istransparent and defects are detectable only when irregularities are seenin the light reflected or refracted by the coated surface. Furthermore,a nonuniform distribution of the adhesive can be produced in particularby the process of removing the moisture film between the plastic sheetand the surface with a squeegee.

SUMMARY OF THE INVENTION

An object of the present invention is to disclose a method of coating asurface with a thin plastic sheet, in particular the surface of anautomobile bumper of the kind described at the outset, by means of whichthe smallest possible number of defective products are delivered. Inthis method any defects that occur are detected as quickly as possible,and wherever possible slight irregularities in the coating areeliminated. Another object of the present invention is to disclose anapparatus for coating a surface with a thin sheet of plastic, by meansof which the above requirements can be fulfilled.

Furthermore, it is an object of the present invention to disclose ameans of employing equipment in such a way as to satisfy the aboverequirements.

An essential idea in the invention presented here is that the coating ofthe surface is irradiated with infrared radiation during and/or afterremoval of the moisture film with a squeegee, in order to warm up atleast portions of the coating and/or, where appropriate, residualmoisture remaining between the surface and the plastic sheet.

When such residual moisture is present, the warming acceleratesprocesses that in the absence of warming would take a longer time tocomplete or would not even be initiated until a later, unpredictabletime when, for example, the coating is exposed to extreme externalinfluences such as a high ambient temperature or intense solarirradiation. Hence in a short time it can be determined whether thecoating satisfies predetermined quality criteria.

The infrared radiation can be absorbed by the plastic sheet itselfand/or by the residual moisture and/or by an adhesive layer, if one ispresent. Preferably, however, the greatest proportion of the incidentenergy is absorbed directly in the region between the surface of theobject to be coated and the plastic sheet. In particular when the periodof irradiation is brief, with high radiation flux densities, the warmingis brought about at the desired place with no appreciable heating of theplastic sheet or the object to be coated. Usually such plastic sheetsare temperature-sensitive, being deformed thermoplastically attemperatures above a damage-level. In the preferred embodiment describedhere, therefore, damage to the plastic sheet and/or the object to becoated can be avoided with high reliability.

Especially preferred is a further development in which a largelycontinuous radiation spectrum is employed, the spectral radiationdensity maximum of which lies in a wavelength range beyond the visibleregion, up to a wavelength of 1.4 μm. This region is called the nearinfrared.

In a preferred further development of the method, any residual moisturethat is present is affected by the infrared radiation in such a way thatit collects to form blisters. Thus only a short time after, or evenduring the irradiation defects in the application of the plastic sheetor in the removal of the moisture film can be detected. These productscan be rejected and/or coated anew. In distinction to the known methoddescribed above, the present method allows quality control to be carriedout immediately after the actual coating process. A later qualitycontrol, e.g. in an automobile production line, can therefore beeliminated, with a saving of work time and personnel costs.

Often an adhesive layer disposed between the surface and the plasticsheet is used to attach the plastic sheet. Preferably in such cases thecoating is irradiated in such a way that the adhesive layer is warmed bythe irradiation and spreads out uniformly. The result is good adhesionof the coating everywhere, with no visually discernible irregularitiescaused by the adhesive layer.

When it is likely that there will be nonuniformities in the plasticsheet, such as an uneven thickness of the sheet or a nonuniformstructure of the plastic material, the irradiation evens them out, atleast approximately, in particular if they are only slightirregularities. In this case an appreciable absorption of the infraredradiation does take place, especially in the plastic sheet, so that ahomogenization, for instance a uniform polymerization, is brought about.However, irregularities can be restricted to the surface of the plasticsheet, so that it is preferable for most of the radiation energy to beabsorbed in this region, producing a smoothing of the sheet surface.

When a car bumper is to be coated with a piece of plastic sheet thatextends approximately over the entire width and length of the bumper,the coating sheet is preferably irradiated for a total time of less than20 s, in particular less than 10 s, in which process the infraredradiation is preferably emitted by a halogen bulb, the incandescentfilament of which has a surface temperature of more than 2500 K.

The apparatus in accordance with the invention for coating a surfacewith a thin plastic sheet, in particular the surface of a car bumper,comprises a radiation source to generate infrared radiation and areflector to increase the flux density of the radiation emitted by thesource and incident on the coating.

The infrared radiation heats at least parts of the coating applied tothe surface, and/or accessory materials used during coating such asliquids or substances added thereto. In particular, the method describedabove can be implemented in at least one of its variants.

In particular when the process of coating the surface of a car bumper iscarried out at a manufacturing station, the apparatus preferablycomprises a holding device to hold the object to be coated and adisplacement device to change the position of a beam emitted from theradiation source and directed onto a particular, limited part of thecoating. By means of the displacement device the radiation beam or theradiation source can be shifted along the surface of the object to becoated, in order to irradiate other parts of the coating as well. Thedisplacement device is advantageously driven by a motor, which inparticular is operated according to predetermined control parameters, sothat the movement of the incident beam occurs according to a specificprogram.

In a further development of the apparatus the displacement devicecomprises a guide element, in particular a rail, along which theradiation source can be shifted. Preferably the guide element is bent toconform to the shape of the object to be coated, so that while theradiation source is displaced, its distance from the surface of theobject to be coated is kept constant. In this case, the control programis advantageously such that the radiation source is displaced along theguide element with constant velocity.

An advantage of the apparatus in accordance with the invention is thatthe object to be coated is not moved as it would be on a productionline, but instead is kept stationary, so that a precise positioning ofthe plastic sheet on the surface to be coated can be carried out whilethe object is at rest. At a specified time the displacement device canbe activated, while the object is still being held by the holdingdevice.

In a preferred embodiment of the apparatus the radiation sourcecomprises at least one halogen bulb, which in particular can be operatedat surface temperatures of more than 2500 K. To generate specialradiation spectra radiation filters, for example a glass pane with thedesired optical properties, are disposed between the radiation sourceand the coating to be irradiated. The sites at which the incandescentfilament of the halogen bulb are attached are preferably cooled by aducted air current. By this means the working life of the halogen bulbcan be distinctly increased. It is also preferable for the reflector tobe kept at or below a predetermined temperature by liquid cooling, tokeep the reflection properties of the reflector constant and to ensurethat the reflector is exposed to as little thermal stress as possible.

In a particularly preferred embodiment the apparatus comprises a controldevice to regulate the coating temperature, including a pyrometer aimedat the coating to measure the temperature thereof. The control device inparticular prevents the plastic sheet from becoming so warm as to bedamaged.

An additional idea in accordance with the invention provides that duringthe implementation of a method for coating a surface with a thin sheetof plastic, in particular according to an embodiment of the methoddescribed above, an infrared bulb is used for quality control, such thatthe coating is irradiated with infrared radiation from the infrared bulbin order to render visible irregularities in the coating and/or toeliminate them, at least approximately. Preferably the infrared bulb isa halogen bulb. In a particular embodiment the infrared bulb is atubular radiator with an elongated incandescent filament extendinglinearly within a tube, in particular a quartz-glass tube, that istransparent to the radiation. The infrared bulb is advantageouslycombined with a reflector that extends in the long direction of the tubeand curves around the back side thereof, so that it has the form of achannel in cross section, with the result that the radiation emittedfrom the front side of the tube is enhanced by reflected radiation. Thusan undesired emission of radiation in the direction of the back side isavoided, and the efficacy of the arrangement is increased. Furthermore,the reflector can serve to provide a particular radiation flux densitydistribution over the irradiated region of the coating, in particular toachieve a constant radiation flux density distribution.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the present invention will now be describedwith reference to the drawing. However, the invention is not restrictedto these exemplary embodiments. The individual figures in the drawingare as follows:

FIG. 1 is an exemplary embodiment of the apparatus in accordance withthe invention,

FIG. 2 shows an example of the structure of a coating before a film ofmoisture between the plastic sheet and the surface to be coated has beenremoved with a squeegee,

FIG. 3 shows the structure of the coating according to FIG. 2 aftertreatment with the squeegee and

FIG. 4 shows the structure of the coating according to FIG. 3immediately after infrared irradiation.

DETAILED DESCRIPTION

FIG. 1 shows an embodiment of the apparatus in accordance with theinvention that comprises a supporting surface 3 onto which a car bumper1 is placed. The car bumper 1 is held by retention clamps 4 that arefixed in position on the supporting surface 3. In this case the bumper 1is part of a safety bumper arrangement to be mounted on the rear end ofan automobile in the region of the door to the luggage compartment. Thebumper 1 is made of a plastic material that is colour-coordinated withthe paint on the body of the car. In particular in order to preventscratching of the bumper 1, by means of the apparatus shown in FIG. 1 athin sheet of polyethylene (PE) is applied to the upper surface of thebumper 1. In FIG. 1 the PE sheet 2 is drawn thicker than it actually is.

The apparatus shown here comprises a rail 5, along which a halogen bulb11 can be shifted. The halogen bulb 11, as can be seen in thecross-sectional drawing, is surrounded on its back side by achannel-shaped reflector 8, which enhances the radiation emitted fromthe front side by adding reflected radiation. The reflector 8 isattached to a bulb holder 6, which in turn is connected to two slides 7that are guided by the rail 5 and permit the halogen bulb 11 to bedisplaced.

The quartz-glass tube and the incandescent filament 10 of the halogenbulb 11 extend perpendicular to the plane of FIG. 1 over a distancecorresponding approximately to the width of the PE sheet 2. Because ofthe combination of the halogen bulb 11 with the reflector 8, a beam 12of radiation that diverges slightly to the sides is incident on a regioncovering part of the coating applied to the bumper 1. By shifting thearrangement along the rail 5, the entire coating can be uniformlyirradiated. The ends of the rail 5 are bent to correspond to the shapeof the bumper 1, so that the end regions of the PE sheet 2 can beirradiated from the same distance as the middle section of the PE sheet2.

The halogen bulb 11 is preferably operated in such a way that thesurface temperature of the incandescent filament 10 amounts to about3000 K. The maximum of the spectral radiation flux density distributionis therefore at approximately 1 μm wavelength, i.e. in the nearinfrared.

With reference to FIGS. 2-4 an exemplary embodiment of the method inaccordance with the invention will now be described, for which inparticular the apparatus according to FIG. 1 can be used. The figuresshow cross sections through a particular restricted part of the bumper 1according to FIG. 1 at various stages of the process.

First the surface of the bumper 1 that is to be coated is wet with anaqueous rinsing solution 16. Then the PE sheet 2, the under surface ofwhich has previously been provided with an adhesive layer 15, is setonto the wet surface. The resulting state is illustrated in FIG. 2. Itcan also happen, in contrast to the situation shown here, or it can evenbe necessary in order to obtain adequate adhesion, for the adhesivelayer 15 to take up moisture, i.e. the rinsing solution, in order toenable the PE sheet to be shifted into precisely the desired position ina simple manner. However, it is advantageous if an excess of rinsingsolution 16 is present, because this prevents or delays the developmentof the attachment action of the adhesive and represents a lubricatingfilm with a low coefficient of friction. After the PE sheet 2 has beenput into place, the rinsing solution is squeezed out, at least to agreat extent, by hand or with a suitable tool. As a result the adhesive15 develops its adhesive action and fixes the PE sheet 2 to the surfaceof the car bumper 1. The condition shown in FIG. 3 is thereby achieved,namely that despite efforts to squeeze out the liquid, in part of thecoated region superfluous rinsing solution 16 still remains on thesurface of the bumper 1, as residual moisture 18. The residual moisture18 is present in a thin layer, because it is bound to the surface of thecar bumper and/or to the adhesive 15 or the lower surface of the PEsheet 2. Furthermore, the adhesive 15 forms a layer of nonuniformthickness, in particular in the region of the residual moisture 18.

Now if one were to wait for a few hours without carrying out any otherprocedural steps, the adhesive 15 would become completely detached fromits binding to the fluid, and in some circumstances would releaseadditional bound particles of the rinsing solution, so that the residualmoisture 18 would gradually accumulate in a confined space and beclearly visible from outside as a blister 17, as shown in FIG. 4. Such acoating does not meet the customary quality criteria, so that the bumper1 must be rejected before being mounted, or if it has already beenmounted on an automobile it would have to be removed and replaced orcoated again.

Hence in accordance with the invention, for quality assurance thecoating of the bumper 1 is irradiated with infrared radiation duringand/or after the moisture has been squeezed out, so that firstly theadhesive 15 becomes distributed in a uniform thickness over the surfaceof the bumper 1 (with the exception of the region where residualmoisture 18 is present) and, secondly, the residual moisture 18 collectsto form a blister 17. The term “blister” is used here for any clearlyvisible elevation of the plastic sheet that is the cause of anaccumulation of liquid; in particular it is also possible for elongated,wavelike blisters to form. Thus already during or shortly after theirradiation it is detectable whether the coating meets the qualitycriteria. Furthermore, in case no superfluous residual moisture is madevisible, it follows that the adhesive layer is uniformly thickeverywhere. In addition, such slight nonuniformities of the plasticsheet as may be present, detectable for example as streaks or cloudlikestructures, can be evened out by the warming.

LIST OF REFERENCE NUMERALS

1 Automobile bumper

2 PE sheet

3 Supporting surface

4 Retaining clamps

5 Rail

6 Bulb holder

7 Slide

8 Reflector

9 Quartz-glass tube

10 Incandescent filament

11 Halogen bulb

12 Radiation beam

15 Adhesive

16 Rinsing solution

17 Blister

18 Residual moisture

What is claimed is:
 1. A method of coating a surface with a plasticsheet comprising: moistening the surface and/or plastic sheet; applyingthe plastic sheet to the surface so that at least a portion of thesurface is separated from the plastic sheet by a film of moisture, theplastic sheet and film of moisture defining a coating; removing at leasta portion of the film of moisture from between the plastic sheet andsurface by squeezing, so that at least one area of residual moistureremains between the surface and the plastic sheet; irradiating thecoating with infrared radiation from a tubular infrared bulb extendingover a distance corresponding approximately to a width of the plasticsheet, the irradiating step occurring during and/or after the removingstep so that at least a portion of the coating and/or the at least onearea of residual moisture is heated.
 2. A method according to claim 1,in which the at least one area of residual moisture forms a blisterduring the irradiating step.
 3. A method according to claim 1, in whichthe coating further comprises an adhesive layer disposed between thesurface and the plastic sheet, and in which the adhesive layer is heatedduring the irradiating step so that the adhesive layer spreads uniformlyover the surface.
 4. A method according to claim 1, in which the plasticsheet includes nonuniformities, and in which the nonuniformities are atleast partially smoothed during the irradiation step.
 5. A methodaccording to claim 1, in which the infrared radiation has a spectralradiation flux density maximum in the near infrared.
 6. A methodaccording to claim 1, in which the coating is irradiated for a total ofless than 20 seconds.
 7. Apparatus for processing a surface having acoating of at least a plastic sheet, the apparatus comprising: a holdingdevice sized to hold an object having the surface to be coated; a sourceof infrared radiation including a tubular bulb extending over a distancecorresponding approximately to a width of the plastic sheet, the bulbpositioned to heat at least a portion of the coating and/or accessorymaterials used in the coating process, the tubular bulb having a frontside facing toward the surface and a back side facing away from thesurface; a reflector positioned adjacent the back side of the tubularbulb to increase a flux density of a radiation beam emitted by thetubular bulb and incident on the coating; and a displacement deviceattached to the source of infrared radiation for shifting the radiationbeam along the surface to irradiate substantially an entire area of thecoating.
 8. Apparatus according to claim 7, wherein the displacementdevice comprises a guide element along which the source of infraredradiation is shifted.
 9. Apparatus according to claim 8, wherein theguide element is bent to correspond to a shape of the object, so that auniform distance is maintained between the source of infrared radiationand the coating as the source of infrared radiation is shifted along theguide element.
 10. Apparatus according to claim 7, wherein the radiationsource comprises at least one halogen bulb operable at a surfacetemperature of at least 2500 K.
 11. Apparatus according to claim 7,further comprising a pyrometer for measuring a coating temperature and acontrol for regulating the coating temperature.
 12. A method ofperforming quality assurance on a coating attached to a surface, thecoating including a plastic sheet, the method comprising: securing thesurface and attached coating with a holding device; generating infraredradiation from a tubular bulb having a front side facing toward thesurface and a back side facing away from the surface; reflectinginfrared radiation emitted from the rear side of the bulb toward thesurface to increase a flux density of the infrared radiation emitted bythe tubular bulb and incident on the coating; and shifting a radiationbeam directed from the tubular bulb along the surface and coating toirradiate substantially an entire area of the coating; wherein theinfrared radiation incident on the coating increases visual detection ofirregularities in the coating and/or reduces the irregularities in thecoating.
 13. A method according to claim 12, wherein the infrared bulbcomprises a halogen bulb.
 14. A method according to claim 12, whereinthe tubular bulb comprises an incandescent filament extending linearlywithin a quartz-glass tube.
 15. A method according to claim 14, whereinthe infrared radiation emitted from the back side of the tubular bulb isreflected by a reflector positioned along a back side of the tube, thereflector having a channel-shaped cross-section open toward the surface.